Orphan nuclear hormone receptor Rev-erbalpha regulates the human apolipoprotein CIII promoter

Downregulation of HNF4A enables transcriptomic reprogramming during the hepatic acute-phase response

The hepatic acute-phase response is characterized by a massive upregulation of serum proteins, such as haptoglobin and serum amyloid A, at the expense of liver homeostatic functions. Although the transcription factor hepatocyte nuclear factor 4 alpha (HNF4A) has a well-established role in safeguarding liver function and its cistrome spans around 50% of liver-specific genes, its role in the acute-phase response has received little attention so far. We demonstrate that HNF4A binds to and represses acute-phase genes under basal conditions. The reprogramming of hepatic transcription during inflammation necessitates loss of HNF4A function to allow expression of acute-phase genes while liver homeostatic genes are repressed. In a pre-clinical liver organoid model overexpression of HNF4A maintained liver functionality in spite of inflammation-induced cell damage. Conversely, HNF4A overexpression potently impaired the acute-phase response by retaining chromatin at regulatory regions of acute-phase genes inaccessible to transcription. Taken together, our data extend the understanding of dual HNF4A action as transcriptional activator and repressor, establishing HNF4A as gatekeeper for the hepatic acute-phase response.

Recent Advances in Hepatic Metabolic Regulation by the Nuclear Factor Rev-erbɑ

Rev-erbɑ (NR1D1) is a nuclear receptor superfamily member that plays a vital role in mammalian molecular clocks and metabolism. Rev-erbɑ can regulate the metabolism of drugs and the body's glucose metabolism, lipid metabolism, and adipogenesis. It is even one of the important regulatory factors regulating the occurrence of metabolic diseases (e.g., diabetes, fatty liver). Metabolic enzymes mediate most drug metabolic reactions in the body. Rev-erbɑ has been recognized to regulate drug metabolic enzymes (such as Cyp2b10 and Ugt1a9). Therefore, this paper mainly reviewed that Rev-erbɑ regulates I and II metabolic enzymes in the liver to affect drug pharmacokinetics. The expression of these drug metabolic enzymes (up-regulated or down-regulated) is related to drug exposure and effects/ toxicity. In addition, our discussion extends to Rev-erbɑ regulating some transporters (such as P-gp, Mrp2, and Bcrp), as they also play an essential role in drug metabolism. Finally, we briefly describe the role and mechanism of nuclear receptor Rev-erbɑ in lipid and glucose homeostasis, obesity, and metabolic disorders syndrome. In conclusion, this paper aims to understand better the role and mechanism of Rev-erbɑ in regulating drug metabolism, lipid, glucose homeostasis, obesity, and metabolic disorders syndrome, which explores how to target Rev-erbɑ to guide the design and development of new drugs and provide scientific reference for the molecular mechanism of new drug development, rational drug use, and drug interaction.

The Role of REV-ERB Receptors in Cancer Pathogenesis

REV-ERB receptors are members of the nuclear receptor superfamily of proteins, which act as both intracellular receptors and transcription factors, therefore modulating the expression of target genes. REV-ERBs act as transcription repressors because of their unique structure. Their predominant role involves the control of peripheral circadian rhythmicity by participating in a transcription-translation feedback loop with other major clock genes. Regarding their role in cancer pathogenesis, recent studies in various cancerous tissues have revealed that their expression was downregulated in the majority of the cases. Dysregulation of their expression was also implicated in cancer-associated cachexia. The pharmacological restoration of their effects is feasible with synthetic agonists, which have been explored in preclinical studies but with scarce data. There is a need for further investigation, primarily with mechanistic studies, on the effect of the REV-ERB-induced circadian rhythm deregulation in carcinogenesis and cancer-related systemic effects, such as cachexia, in order to address the potential of relevant therapeutic implications.

The REV-ERB Nuclear Receptors: Timekeepers for the Core Clock Period and Metabolism

REV-ERB nuclear receptors are potent transcriptional repressors that play an important role in the core mammalian molecular clock and metabolism. Deletion of both REV-ERBα and its largely redundant isoform REV-ERBβ in a murine tissue-specific manner have shed light on their specific functions in clock mechanisms and circadian metabolism. This review highlights recent findings that establish REV-ERBs as crucial circadian timekeepers in a variety of tissues, regulating overlapping and distinct processes that maintain normal physiology and protect from metabolic dysfunction.

Combination of Genomic and Transcriptomic Approaches Highlights Vascular and Circadian Clock Components in Multiple Sclerosis

Aiming at exploring vascular components in multiple sclerosis (MS) with brain outflow disturbance, we combined transcriptome analysis in MS internal jugular vein (IJV) wall with WES in MS families with vertical transmission of disease. Main results were the differential expression in IJV wall of 16 MS-GWAS genes and of seven genes (GRIN2A, GRIN2B, IL20RB, IL26, PER3, PITX2, and PPARGC1A) not previously indicated by GWAS but encoding for proteins functionally interacting with MS candidate gene products. Strikingly, 22/23 genes have been previously associated with vascular or neuronal traits/diseases, nine encoded for transcriptional factors/regulators and six (CAMK2G, GRIN2A, GRIN2B, N1RD1, PER3, PPARGC1A) for circadian entrainment/rhythm components. Among the WES low-frequency (MAF ≤ 0.04) SNPs (n = 7) filtered in the 16 genes, the NR1D1 rs17616365 showed significantly different MAF in the Network for Italian Genomes affected cohort than in the 1000 Genome Project Tuscany samples. This pattern was also detected in five nonintronic variants (GRIN2B rs1805482, PER3 rs2640909, PPARGC1A rs2970847, rs8192678, and rs3755863) in genes coding for functional partners. Overall, the study proposes specific markers and low-frequency variants that might help (i) to understand perturbed biological processes in vascular tissues contributing to MS disease, and (ii) to characterize MS susceptibility genes for functional association with disease-pathways.

Nuclear Receptors and Clock Components in Cardiovascular Diseases

Cardiovascular diseases (CVD) are still the first cause of death worldwide. Their main origin is the development of atherosclerotic plaque, which consists in the accumulation of lipids and inflammatory leucocytes within the vascular wall of large vessels. Beyond dyslipidemia, diabetes, obesity, hypertension and smoking, the alteration of circadian rhythms, in shift workers for instance, has recently been recognized as an additional risk factor. Accordingly, targeting a pro-atherogenic pathway at the right time window, namely chronotherapy, has proven its efficiency in reducing plaque progression without affecting healthy tissues in mice, thus providing the rationale of such an approach to treat CVD and to reduce drug side effects. Nuclear receptors are transcriptional factors involved in the control of many physiological processes. Among them, Rev-erbs and RORs control metabolic homeostasis, inflammatory processes and the biological clock. In this review, we discuss the opportunity to dampen atherosclerosis progression by targeting such ligand-activated core clock components in a (chrono-)therapeutic approach in order to treat CVD.

Bisphenol A attenuates testosterone production in Leydig cells via the inhibition of NR1D1 signaling

Bisphenol A (BPA) is an endocrine-disrupting compound that impairs testosterone synthesis in male mammals. A circadian clock gene deficiency leads to diminished fertility and even infertility in male mice. However, whether circadian clock signaling pathways mediate the suppressive effect of BPA on testosterone synthesis in Leydig cells (LCs) remains unknown. The present study aims to detect the effect of BPA on cellular circadian clock and testosterone synthesis in mouse LCs, and examine the mechanisms underlying NR1D1 signaling. BPA treatment significantly attenuated the transcription levels of Nr1d1 and steroidogenic genes (Hsd3b2 and Hsd17b3) in TM3 cells, but increased other circadian clock gene levels (Per2 and Dbp). BPA treatment also significantly downregulated NR1D1 and StAR protein expression, but upregulated BMAL1 protein expression in TM3 cells. Furthermore, there was a marked decline in testosterone production in BPA-treated TM3 cells. Intraperitoneal injection of BPA profoundly reduced NR1D1 and StAR protein levels and steroidogenic gene transcription levels (Cyp11a1, Hsd3b2, and Hsd17b3), while enhancing BMAL1 protein and other circadian clock gene (Per2 and Dbp) levels in mouse testes. Notably, serum testosterone levels were also drastically reduced in BPA-treated mice. Moreover, SR9009, an NR1D1 agonist, augmented testosterone production in TM3 cells via elevated expression of steroidogenic genes (StAR, Cyp11a1 and Hsd17b3). Conversely, Nr1d1 knockdown inhibited testosterone accumulation and attenuated steroidogenic gene expression. Moreover, treatment with SR9009 partially reversed the BPA effect on the circadian clock and testosterone production. Taken together, our study demonstrates that BPA perturbs testosterone production, at least partially, via inhibiting NR1D1 signaling in LCs.

Apolipoprotein C-III and cardiovascular diseases: when genetics meet molecular pathologies

Cardiovascular diseases (CVD) have overtaken infectious diseases and are currently the world's top killer. A quite strong linkage between this type of ailments and elevated plasma levels of triglycerides (TG) has been always noticed. Notably, this risk factor is mired in deep confusion, since its role in atherosclerosis is uncertain. One of the explanations that aim to decipher this persistent enigma was provided by apolipoprotein C-III (apoC-III), a small protein historically recognized as an important regulator of TG metabolism. Preeminently, hundreds of studies have been carried out in order to explore the APOC3 genetic background, as well as to establish a correlation between its variants and dyslipidemia-related disorders, pointing to an earnest predictive power for future outcomes. Among several polymorphisms reported within the APOC3, the SstI site in its 3'-untranslated region (3'-UTR) was the most consistently and robustly associated with an increased CVD risk. As more genetic data supporting its importance in cardiovascular events aggregate, it was declared, correspondingly, that apoC-III exerts various atherogenic effects, either by intervening in the function and catabolism of many lipoproteins, or by inducing endothelial inflammation and smooth muscle cells (SMC) proliferation. This review was designed to shed the light on the structural and functional aspects of the APOC3 gene, the existing association between its SstI polymorphism and CVD, and the specific molecular mechanisms that underlie apoC-III pathological implications. In addition, the translation of all these gathered knowledges into preventive and therapeutic benefits will be detailed too.

Identifying Transcription Factor Combinations to Modulate Circadian Rhythms by Leveraging Virtual Knockouts on Transcription Networks

The mammalian circadian systems consist of indigenous, self-sustained 24-h rhythm generators. They comprise many genes, molecules, and regulators. To decode their systematic controls, a robust computational approach was employed. It integrates transcription-factor-occupancy and time-series gene-expression data as input. The model equations were constructed and solved to determine the transcriptional regulatory logics in the mouse transcriptome network. This hypothesizes to explore the underlying mechanisms of combinatorial transcriptional regulations for circadian rhythms in mouse. We reconstructed the quantitative transcriptional-regulatory networks for circadian gene regulation at a dynamic scale. Transcriptional-simulations with virtually knocked-out mutants were performed to estimate their influence on networks. The potential transcriptional-regulators-combinations modulating the circadian rhythms were identified. Of them, CLOCK/CRY1 double knockout preserves the highest modulating capacity. Our quantitative framework offers a quick, robust, and physiologically relevant way to characterize the druggable targets to modulate the circadian rhythms at a dynamic scale effectively.

Coordination between the circadian clock and androgen signaling is required to sustain rhythmic expression of Elovl3 in mouse liver

ELOVL3 is a very long-chain fatty acid elongase, and its mRNA levels display diurnal rhythmic changes exclusively in adult male mouse livers. This cyclical expression of hepatic Elovl3 is potentially controlled by the circadian clock, related hormones, and transcriptional factors. It remains unknown, however, whether the circadian clock, in conjunction with androgen signaling, functions in maintaining the rhythmic expression of Elovl3 in a sexually dimorphic manner. Under either zeitgeber or circadian time, WT mouse livers exhibited a robust circadian rhythmicity in the expression of circadian clock genes and Elovl3 In contrast, male Bmal1-/- mice displayed severely weakened expression of hepatic circadian clock genes, resulting in relatively high, but nonrhythmic, Elovl3 expression levels. ChIP assays revealed that NR1D1 binds to the Elovl3 promoter upon circadian change in WT mouse livers in vivo, and a diminished binding was observed in male Bmal1-/- mouse livers. Additionally, female mouse livers exhibited constant low levels of Elovl3 expression. Castration markedly reduced Elovl3 expression levels in male mouse livers but did not disrupt circadian variation of Elovl3 Injection of female mice with 5α-dihydrotestosterone induced Elovl3 rhythmicity in the liver. In AML12 cells, 5α-dihydrotestosterone also elevated Elovl3 expression in a time-dependent manner. In contrast, flutamide efficiently attenuated this induction effect. In conclusion, a lack of either the circadian clock or androgen signaling impairs hepatic Elovl3 expression, highlighting the observation that coordination between the circadian clock and androgen signaling is required to sustain the rhythmic expression of Elovl3 in mouse liver.

Genome-Wide Linkage Analysis of Large Multiple Multigenerational Families Identifies Novel Genetic Loci for Coronary Artery Disease

Coronary artery disease (CAD) is the leading cause of death, and genetic factors contribute significantly to risk of CAD. This study aims to identify new CAD genetic loci through a large-scale linkage analysis of 24 large and multigenerational families with 433 family members (GeneQuest II). All family members were genotyped with markers spaced by every 10 cM and a model-free nonparametric linkage (NPL-all) analysis was carried out. Two highly significant CAD loci were identified on chromosome 17q21.2 (NPL score of 6.20) and 7p22.2 (NPL score of 5.19). We also identified four loci with significant NPL scores between 4.09 and 4.99 on 2q33.3, 3q29, 5q13.2 and 9q22.33. Similar analyses in individual families confirmed the six significant CAD loci and identified seven new highly significant linkages on 9p24.2, 9q34.2, 12q13.13, 15q26.1, 17q22, 20p12.3, and 22q12.1, and two significant loci on 2q11.2 and 11q14.1. Two loci on 3q29 and 9q22.33 were also successfully replicated in our previous linkage analysis of 428 nuclear families. Moreover, two published risk variants, SNP rs46522 in UBE2Z and SNP rs6725887 in WDR12 by GWAS, were found within the 17q21.2 and 2q33.3 loci. These studies lay a foundation for future identification of causative variants and genes for CAD.

Genetic risk variants for metabolic traits in Arab populations

Despite a high prevalence of metabolic trait related diseases in Arabian Peninsula, there is a lack of convincingly identified genetic determinants for metabolic traits in this population. Arab populations are underrepresented in global genome-wide association studies. We genotyped 1965 unrelated Arab individuals from Kuwait using Cardio-MetaboChip, and tested SNP associations with 13 metabolic traits. Models based on recessive mode of inheritance identified Chr15:40531386-rs12440118/ZNF106/W->R as a risk variant associated with glycated-hemoglobin at close to ‘genome-wide significant’ p-value and five other risk variants ‘nominally’ associated (p-value ≤ 5.45E-07) with fasting plasma glucose (rs7144734/[OTX2-AS1,RPL3P3]) and triglyceride (rs17501809/PLGRKT; rs11143005/LOC105376072; rs900543/[THSD4,NR2E3]; and Chr12:101494770/IGF1). Furthermore, we identified 33 associations (30 SNPs with 12 traits) with ‘suggestive’ evidence of association (p-value < 1.0E-05); 20 of these operate under recessive mode of inheritance. Two of these ‘suggestive’ associations (rs1800775-CETP/HDL; and rs9326246-BUD13/TGL) showed evidence at genome-wide significance in previous studies on Euro-centric populations. Involvement of many of the identified loci in mediating metabolic traits was supported by literature evidences. The identified loci participate in critical metabolic pathways (such as Ceramide signaling, and Mitogen-Activated Protein Kinase/Extracellular Signal Regulated Kinase signaling). Data from Genotype-Tissue Expression database affirmed that 7 of the identified variants differentially regulate the up/downstream genes that mediate metabolic traits.

Apoprotein C-III: A review of its clinical implications

Apoprotein C-III (apoC-III), originating from the apoA-I/C-III/A-IV gene cluster affected by multiple regulating factors, has been demonstrated to have a validated link with hypertriglyceridemia in humans. Following genome studies establishing the impact of apoC-III on both plasma triglyceride (TG) level and cardiovascular disease (CVD), apoC-III offers us a novel explanation attempting to resolve the long-existing confusion with regard to the atherogenic effect of TG. Notably, apoC-III exerts its atherogenic effect by means of not only intervening in the function and metabolism of various lipid molecules, but also accelerating pro-inflammatory effects between monocytes and endothelial cells. Data have suggested that diabetes, a common endocrine disease, also correlates closely with apoC-III in its apoptosis process of islet βcells. In fact, apoC-III genes, with various mutations among individuals, are also found to have relevance to other diseases, including fatty liver disease. Fortunately, besides present day therapeutic strategies, such as lifestyle changes and lipid-lowering drug treatments, a promising new antisense drug specifically targeting on apoC-III gene expression opens up new avenues. This article mainly summarizes the clinical implication of apoC-III and its future directions of treatment.

RNA Dynamics in the Control of Circadian Rhythm

The circadian oscillator is based on transcription-translation feedback loops that generate 24 h oscillations in gene expression. Although circadian regulation of mRNA expression at the transcriptional level is one of the most important steps for the generation of circadian rhythms within the cell, multiple lines of evidence point to a disconnect between transcript oscillation and protein oscillation. This can be explained by regulatory RNA-binding proteins acting on the nascent transcripts to modulate their processing, export, translation and degradation rates. In this chapter we will review what is known about the different steps involved in circadian gene expression from transcription initiation to mRNA stability and translation efficiency. The role of ribonucleoprotein particles in the generation of rhythmic gene expression is only starting to be elucidated, but it is likely that they cooperate with the basal transcriptional machinery to help to maintain the precision of the clock under diverse cellular and environmental conditions.

High Affinity Heme Binding to a Heme Regulatory Motif on the Nuclear Receptor Rev-erbβ Leads to Its Degradation and Indirectly Regulates Its Interaction with Nuclear Receptor Corepressor

Rev-erbα and Rev-erbβ are heme-binding nuclear receptors (NR) that repress the transcription of genes involved in regulating metabolism, inflammation, and the circadian clock. Previous gene expression and co-immunoprecipitation studies led to a model in which heme binding to Rev-erbα recruits nuclear receptor corepressor 1 (NCoR1) into an active repressor complex. However, in contradiction, biochemical and crystallographic studies have shown that heme decreases the affinity of the ligand-binding domain of Rev-erb NRs for NCoR1 peptides. One explanation for this discrepancy is that the ligand-binding domain and NCoR1 peptides used for in vitro studies cannot replicate the key features of the full-length proteins used in cellular studies. However, the combined in vitro and cellular results described here demonstrate that heme does not directly promote interactions between full-length Rev-erbβ (FLRev-erbβ) and an NCoR1 construct encompassing all three NR interaction domains. NCoR1 tightly binds both apo- and heme-replete FLRev-erbβ·DNA complexes; furthermore, heme, at high concentrations, destabilizes the FLRev-erbβ·NCoR1 complex. The interaction between FLRev-erbβ and NCoR1 as well as Rev-erbβ repression at the Bmal1 promoter appear to be modulated by another cellular factor(s), at least one of which is related to the ubiquitin-proteasome pathway. Our studies suggest that heme is involved in regulating the degradation of Rev-erbβ in a manner consistent with its role in circadian rhythm maintenance. Finally, the very slow rate constant (10(-6) s(-1)) of heme dissociation from Rev-erbβ rules out a prior proposal that Rev-erbβ acts as an intracellular heme sensor.

Bringing obesity to light: Rev-erbα, a central player in light-induced adipogenesis in the zebrafish?

BACKGROUND

Recent studies have led to an expansion of potential factors capable of stimulating obesity. Increasing evidence indicates that environmental factors, including disturbance of circadian rhythms, also contribute to its etiology.

OBJECTIVES

To determine the effects of altered circadian rhythms on adipogenesis and to better understand how circadian and adipogenic regulatory pathways are linked, zebrafish larvae were exposed to various light/dark cycles or hypercaloric feeding (HCF).

METHODS

Clock and adipogenic gene expression was quantitative real time PCR. Adipogenesis was characterized using coherent anti-Stokes Raman scattering microscopy (CARS) and whole-mount lipid composition was analyzed by gas chromatography. The clock protein Rev-erbα and the adipogenesis-regulating protein Pparγ were localized by immunohistochemistry.

RESULTS

Zebrafish larvae exposed to continuous light (LL) had a sevenfold higher prevalence of adipocytes compared with control fish under a 14 h light and 10 h dark cycle. It was also significantly higher compared with that in HCF larvae with control light/dark cycle, which showed a 5.5-fold increase compared with control animals. Although total fatty acid content was unaffected, adipocyte lipid composition was altered in LL zebrafish. In contrast, shifting the onset and duration of the light periods did not affect adipogenesis or total fatty acid content. Gene expression analysis revealed effects of LL and HCF on circadian cyclicity, with increased expression of the clock gene period2 and altered circadian rev-erbα expression in LL larvae. Immunostaining revealed for the first time that Rev-erbα and Pparγ colocalize in adipocytes, which together with the gene expression analysis suggests interplay between Rev-erbα and Ppar isoforms.

CONCLUSIONS

The amount of light, but not shifted light/dark cycles, affected adipogenesis and lipid composition, possibly due to increased period2 expression, which, in turn, enhances Rev-erbα-regulated gene expression. As the pparβδ promoter includes three Rev-erbα binding sites, we hypothesize that pparβδ may be a direct target that ultimately activates Pparγ.

Molecular mechanisms of transcriptional control by Rev-erbα: An energetic foundation for reconciling structure and binding with biological function

Rev-erbα and β are nuclear receptors that function as transcriptional repressors of genes involved in regulating circadian rhythms, glucose, and cholesterol metabolism and the inflammatory response. Given these key functions, Rev-erbs are important drug targets for treatment of a number of human pathologies, including cancer, heart disease, and type II diabetes. Transcriptional repression by the Rev-erbs involves direct competition with transcriptional activators for target sites, but also recruitment by the Rev-erbs of the NCoR corepressor protein. Interestingly, Rev-erbs do not appear to interact functionally with a very similar corepressor, Smrt. Transcriptional repression by Rev-erbs is thought to occur in response to the binding of heme, although structural, and ligand binding studies in vitro show that heme and corepressor binding are antagonistic. We carried out systematic studies of the ligand and corepressor interactions to address the molecular basis for corepressor specificity and the energetic consequences of ligand binding using a variety of biophysical approaches. Highly quantitative fluorescence anisotropy assays in competition mode revealed that the Rev-erb specificity for the NCoR corepressor lies in the first two residues of the β-strand in Interaction Domain 1 of NCoR. Our studies confirmed and quantitated the strong antagonism of heme and corepressor binding and significant stabilization of the corepressor complex by a synthetic ligand in vitro. We propose a model which reconciles the contradictory observations concerning the effects of heme binding in vitro and in live cells.

Plasma apolipoprotein C-III levels, triglycerides, and coronary artery calcification in type 2 diabetics

OBJECTIVE

Triglyceride-rich lipoproteins have emerged as causal risk factors for developing coronary heart disease independent of low-density lipoprotein cholesterol levels. Apolipoprotein C-III (ApoC-III) modulates triglyceride-rich lipoprotein metabolism through inhibition of lipoprotein lipase and hepatic uptake of triglyceride-rich lipoproteins. Mutations causing loss-of-function of ApoC-III lower triglycerides and reduce coronary heart disease risk, suggestive of a causal role for ApoC-III. Little data exist about the relationship of ApoC-III, triglycerides, and atherosclerosis in patients with type 2 diabetes mellitus (T2DM). Here, we examined the relationships between plasma ApoC-III, triglycerides, and coronary artery calcification in patients with T2DM.

APPROACH AND RESULTS

Plasma ApoC-III levels were measured in a cross-sectional study of 1422 subjects with T2DM but without clinically manifest coronary heart disease. ApoC-III levels were positively associated with total cholesterol (Spearman r=0.36), triglycerides (r=0.59), low-density lipoprotein cholesterol (r=0.16), fasting glucose (r=0.16), and glycosylated hemoglobin (r=0.12; P<0.0001 for all). In age, sex, and race-adjusted analysis, ApoC-III levels were positively associated with coronary artery calcification (Tobit regression ratio, 1.78; 95% confidence interval, 1.27-2.50 per SD increase in ApoC-III; P<0.001). As expected for an intermediate mediator, these findings were attenuated when adjusted for both triglycerides (Tobit regression ratio, 1.43; 95% confidence interval, 0.94-2.18; P=0.086) and separately for very low-density lipoprotein cholesterol (Tobit regression ratio, 1.14; 95% confidence interval, 0.75-1.71; P=0.53).

CONCLUSIONS

In persons with T2DM, increased plasma ApoC-III is associated with higher triglycerides, less favorable cardiometabolic phenotypes, and higher coronary artery calcification, a measure of subclinical atherosclerosis. Therapeutic inhibition of ApoC-III may thus be a novel strategy for reducing plasma triglyceride-rich lipoproteins and cardiovascular risk in T2DM.

Nuclear receptor Rev-erbα: up, down, and all around

Rev-erbα is a nuclear receptor that links circadian rhythms to transcriptional control of metabolic pathways. Rev-erbα is a potent transcriptional repressor and plays an important role in the core mammalian molecular clock while also serving as a key regulator of clock output in metabolic tissues including liver and brown adipose tissue (BAT). Recent findings have shed new light on the role of Rev-erbα and its paralog Rev-erbβ in rhythm generation, as well as additional regulatory roles for Rev-erbα in other tissues that contribute to energy expenditure, inflammation, and behavior. This review highlights physiological functions of Rev-erbα and β in multiple tissues and discusses the therapeutic potential and challenges of targeting these pathways in human disease.

The therapeutic potential of nuclear receptor modulators for treatment of metabolic disorders: PPARγ, RORs, and Rev-erbs

Nuclear receptors (NRs) play central roles in metabolic syndrome, making them attractive drug targets despite the challenge of achieving functional selectivity. For instance, members of the thiazolidinedione class of insulin sensitizers offer robust efficacy but have been limited due to adverse effects linked to activation of genes not involved in insulin sensitization. Studies reviewed here provide strategies for targeting subsets of PPARγ target genes, enabling development of next-generation modulators with improved therapeutic index. Additionally, emerging evidence suggests that targeting the NRs ROR and Rev-erb holds promise for treating metabolic syndrome based on their involvement in circadian rhythm and metabolism.

Modulation of nuclear receptor function by cellular redox poise

Nuclear receptors (NRs) are ligand-responsive transcription factors involved in diverse cellular processes ranging from metabolism to circadian rhythms. This review focuses on NRs that contain redox-active thiol groups, a common feature within the superfamily. We will begin by describing NRs, how they regulate various cellular processes and how binding ligands, corepressors and/or coactivators modulate their activity. We will then describe the general area of redox regulation, especially as it pertains to thiol-disulfide interconversion and the cellular systems that respond to and govern this redox equilibrium. Lastly, we will discuss specific examples of NRs whose activities are regulated by redox-active thiols. Glucocorticoid, estrogen, and the heme-responsive receptor, Rev-erb, will be described in the most detail as they exhibit archetypal redox regulatory mechanisms.

Increased atherosclerotic lesions in LDL receptor deficient mice with hematopoietic nuclear receptor Rev-erbα knock- down

BACKGROUND

Nuclear receptor Rev-erbα plays important roles in circadian clock timing, lipid metabolism, adipogenesis, and vascular inflammation. However, the role of Rev-erbα in atherosclerotic lesion development has not been assessed in vivo.

METHODS AND RESULTS

The nuclear receptor Rev-erbα was knocked down in mouse haematopoietic cells by means of shRNA-lentiviral transduction, followed by bone marrow transplantation into LDL receptor knockout mice. The Rev-erbα protein in peripheral macrophage was reduced by 70% as compared to control mice injected with nontargeting shRNA lentivirus-transduced bone marrow. A significant increase in atherosclerotic lesions was observed around the aorta valves as well as upon en face aorta analysis of Rev-erbα knock-down bone marrow recipients (P<0.01) as compared to the control mice, while plasma cholesterol, phospholipid, and triacylglycerol levels were not affected. Overexpression of Rev-erbα in bone marrow mononuclear cells decreased inflammatory M1 while increasing M2 macrophage markers, while Rev-erbα knock down increased the macrophage inflammatory phenotype in vitro and in vivo. Furthermore, treatment of differentiating macrophages with the Rev-erbα ligand heme promoted expression of antiinflammatory M2 markers.

CONCLUSIONS

These observations identify hematopoietic cell Rev-erbα as a new modulator of atherogenesis in mice.

Impact of REV-ERB alpha gene polymorphisms on obesity phenotypes in adult and adolescent samples

BACKGROUND

REV-ERBα has been shown to regulate adipogenesis and lipid metabolism as well as to link the circadian timing system to whole body metabolic homeostasis. We thus tested whether polymorphisms in REV-ERBα could be associated with metabolic phenotypes in human population samples.

METHODS

We analyzed the associations between 5 REV-ERBα polymorphisms and anthropometric (body weight, body mass index (BMI), waist and hip circumferences), biochemical (plasma lipid, glucose and insulin levels) and clinical (systolic and diastolic blood pressure) variables in three population-based studies (MONICA Lille n=1155 adults, MONA LISA Lille n=1170 adults and HELENA n=1155 adolescents). We assessed in vitro, the potential influence of one REV-ERBα polymorphism in transient transfection assays using two different cell lines.

RESULTS

We observed significant and consistent associations between the T minor allele of the REV-ERBα rs2071427 polymorphism (located in intron 1) and higher BMI (mean allele effect=+0.33 kg m(-2)) in the MONICA Lille (P=0.02), MONA LISA (P=0.02) and HELENA (P=0.03) studies. The odds ratios for obesity associated with this allele were 1.67 (1.00-2.79) (P=0.05) in MONICA Lille, 1.29 (1.01-1.65) (P=0.04) in MONA LISA Lille and the odds ratio for overweight was 1.48 (1.08-2.03) (P=0.01) in HELENA. In transfection experiments in human hepatocyte-derived cell lines, the REV-ERBα intron 1 directed the transcription of a luciferase reporter gene independently of the rs2071427 polymorphism.

CONCLUSION

Our results suggest that the REV-ERBα rs2071427 polymorphism modulates body fat mass in both adult and young people.

Nutrient sensing and the circadian clock

The circadian system synchronizes behavioral and physiologic processes with daily changes in the external light-dark cycle, optimizing energetic cycles with the rising and setting of the sun. Molecular clocks are organized hierarchically, with neural clocks orchestrating the daily switch between periods of feeding and fasting, and peripheral clocks generating 24h oscillations of energy storage and utilization. Recent studies indicate that clocks respond to nutrient signals and that a high-fat diet influences the period of locomotor activity under free-running conditions, a core property of the clock. A major goal is to identify the molecular basis for the reciprocal relation between metabolic and circadian pathways. Here the role of peptidergic hormones and macromolecules as nutrient signals integrating circadian and metabolic systems is highlighted.

Retinoic acid-related orphan receptor-α is induced in the setting of DNA damage and promotes pulmonary emphysema

RATIONALE

The discovery that retinoic acid-related orphan receptor (Rora)-α is highly expressed in lungs of patients with COPD led us to hypothesize that Rora may contribute to the pathogenesis of emphysema.

OBJECTIVES

To determine the role of Rora in smoke-induced emphysema.

METHODS

Cigarette smoke extract in vitro and elastase or cigarette smoke exposure in vivo were used to model smoke-related cell stress and airspace enlargement. Lung tissue from patients undergoing lung transplantation was examined for markers of DNA damage and Rora expression.

MEASUREMENTS AND MAIN RESULTS

Rora expression was induced by cigarette smoke in mice and in cell culture. Gene expression profiling of Rora-null mice exposed to cigarette smoke demonstrated enrichment for genes involved in DNA repair. Rora expression increased and Rora translocated to the nucleus after DNA damage. Inhibition of ataxia telangiectasia mutated decreased the induction of Rora. Gene silencing of Rora attenuated apoptotic cell death in response to cigarette smoke extract, whereas overexpression of Rora enhanced apoptosis. Rora-deficient mice were protected from elastase and cigarette smoke induced airspace enlargement. Finally, lungs of patients with COPD showed evidence of increased DNA damage even in the absence of active smoking.

CONCLUSIONS

Taken together, these findings suggest that DNA damage may contribute to the pathogenesis of emphysema, and that Rora has a previously unrecognized role in cellular responses to genotoxicity. These findings provide a potential link between emphysema and features of premature ageing, including enhanced susceptibility to lung cancer.

Apolipoprotein C-III and hepatic triglyceride-rich lipoprotein production

PURPOSE OF REVIEW

A strong positive correlation between plasma apolipoprotein (apo) C-III and triglyceride concentrations has been invariably observed in human and animal studies. The hypertriglyceridemic effect of apo C-III has been conventionally explained by its extracellular roles in inhibiting lipolysis catalysed by lipoprotein lipase and attenuating triglyceride-rich lipoprotein clearance through receptor-dependent and/or independent mechanisms. However, recent experimental evidence suggests that apo C-III may also play an intracellular role in promoting hepatic triglyceride-rich lipoprotein production.

RECENT FINDINGS

Kinetic studies with humans and genetically modified mice have shown that apo C-III is linked with increased production of triglyceride-rich lipoproteins, such as very-low-density lipoprotein 1 (VLDL1). Mutational studies on human apo C-III variants (originally identified in humans with hypotriglyceridemia or hyperalphalipoproteinemia) provide the structure-function analysis of human apo C-III, demonstrating that loss-of-function mutations within human apo C-III impair the assembly and secretion of triglyceride-rich VLDL1 under lipid-rich conditions.

SUMMARY

The current review summarizes recent experimental evidence for an intrahepatic role of human apo C-III in promoting mobilization and utilization of triglyceride during VLDL1 assembly/secretion. Understanding mechanisms by which hepatic apo C-III expression is regulated under insulin resistance and diabetic conditions will lead to better and more rational strategies for the prevention and treatment of diabetic hypertriglyceridemia that is closely related to premature atherosclerosis.

A Period 2 genetic variant interacts with plasma SFA to modify plasma lipid concentrations in adults with metabolic syndrome

Genetic variants of Period 2 (PER2), a circadian clock gene, have been linked to metabolic syndrome (MetS). However, it is still unknown whether these genetic variants interact with the various types of plasma fatty acids. This study investigated whether common single nucleotide polymorphisms (SNPs) in the PER2 locus (rs934945 and rs2304672) interact with various classes of plasma fatty acids to modulate plasma lipid metabolism in 381 participants with MetS in the European LIPGENE study. Interestingly, the rs2304672 SNP interacted with plasma total SFA concentrations to affect fasting plasma TG, TG-rich lipoprotein (TRL-TG), total cholesterol, apoC-II, apoB, and apoB-48 concentrations (P-interaction < 0.001-0.046). Carriers of the minor allele (GC+GG) with the highest SFA concentration (>median) had a higher plasma TG concentration (P = 0.001) and higher TRL-TG (P < 0.001) than the CC genotype. In addition, participants carrying the minor G allele for rs2304672 SNP and with a higher SFA concentration (>median) had higher plasma concentrations of apo C-II (P < 0.001), apo C-III (P = 0.009), and apoB-48 (P = 0.028) compared with the homozygotes for the major allele (CC). In summary, the rs2304672 polymorphism in the PER2 gene locus may influence lipid metabolism by interacting with the plasma total SFA concentration in participants with MetS. The understanding of these gene-nutrient interactions could help to provide a better knowledge of the pathogenesis in MetS.

Structural analysis of heme proteins: implications for design and prediction

BACKGROUND

Heme is an essential molecule and plays vital roles in many biological processes. The structural determination of a large number of heme proteins has made it possible to study the detailed chemical and structural properties of heme binding environment. Knowledge of these characteristics can provide valuable guidelines in the design of novel heme proteins and help us predict unknown heme binding proteins.

RESULTS

In this paper, we constructed a non-redundant dataset of 125 heme-binding protein chains and found that these heme proteins encompass at least 31 different structural folds with all-α class as the dominating scaffold. Heme binding pockets are enriched in aromatic and non-polar amino acids with fewer charged residues. The differences between apo and holo forms of heme proteins in terms of the structure and the binding pockets have been investigated. In most cases the proteins undergo small conformational changes upon heme binding. We also examined the CP (cysteine-proline) heme regulatory motifs and demonstrated that the conserved dipeptide has structural implications in protein-heme interactions.

CONCLUSIONS

Our analysis revealed that heme binding pockets show special features and that most of the heme proteins undergo small conformational changes after heme binding, suggesting the apo structures can be used for structure-based heme protein prediction and as scaffolds for future heme protein design.

Nuclear Receptors: Small Molecule Sensors that Coordinate Growth, Metabolism and Reproduction

One of the largest groups of metazoan transcription factors (TFs), the Nuclear Receptor superfamily, regulates genes required for virtually all aspects of development, reproduction and metabolism. Together, these master regulators can be thought of as a fundamental operating system for metazoan life. Their most distinguishing feature is a structurally conserved domain that acts as a switch, powered by the presence of small diffusible ligands. This ligand-responsive regulation has allowed the Nuclear Receptors to help their hosts adapt to a wide variety of physiological niches and roles, making them one of the most evolutionarily successful TF families. Originally discovered as receptors for steroid hormones, the Nuclear Receptor field has grown to encompass much more than traditional endocrinology. For example, recent work has highlighted the role of Nuclear Receptors as major regulators of metabolism and biological clocks. By monitoring endogenous metabolites and absorbed xenobiotics, these receptors also coordinate rapid, system-wide responses to changing metabolic and environmental states. While many new Nuclear Receptor ligands have been discovered in the past couple of decades, approximately half of the 48 human receptors are still orphans, with a significantly higher percentage of orphans in other organisms. The discovery of new ligands has led to the elucidation of new regulatory mechanisms, target genes, pathways and functions. This review will highlight both the common as well as newly emerging traits and functions that characterize this particularly unique and important TF family.

Transcriptional activation of apolipoprotein CIII expression by glucose may contribute to diabetic dyslipidemia

OBJECTIVE

Hypertriglyceridemia and fatty liver are common in patients with type 2 diabetes, but the factors connecting alterations in glucose metabolism with plasma and liver lipid metabolism remain unclear. Apolipoprotein CIII (apoCIII), a regulator of hepatic and plasma triglyceride metabolism, is elevated in type 2 diabetes. In this study, we analyzed whether apoCIII is affected by altered glucose metabolism.

METHODS AND RESULTS

Liver-specific insulin receptor-deficient mice display lower hepatic apoCIII mRNA levels than controls, suggesting that factors other than insulin regulate apoCIII in vivo. Glucose induces apoCIII transcription in primary rat hepatocytes and immortalized human hepatocytes via a mechanism involving the transcription factors carbohydrate response element-binding protein and hepatocyte nuclear factor-4α. ApoCIII induction by glucose is blunted by treatment with agonists of farnesoid X receptor and peroxisome proliferator-activated receptor-α but not liver X receptor, ie, nuclear receptors controlling triglyceride metabolism. Moreover, in obese humans, plasma apoCIII protein correlates more closely with plasma fasting glucose and glucose excursion after oral glucose load than with insulin.

CONCLUSIONS

Glucose induces apoCIII transcription, which may represent a mechanism linking hyperglycemia, hypertriglyceridemia, and cardiovascular disease in type 2 diabetes.

Analysis of the molecular and regulatory properties of active porcine endogenous retrovirus gamma-1 long terminal repeats in kidney tissues of the NIH-Miniature pig

The pig genome contains the gamma 1 family of porcine endogenous retroviruses (PERVs), which are a major obstacle to the development of successful xenotransplantation from pig to human. Long terminal repeats (LTRs) found in PERVs are known to be essential elements for the control of the transcriptional activity of single virus by different transcription factors (TFs). To identify transcribed PERV LTR elements, RT-PCR and DNA sequencing analyses were performed. Twenty-nine actively transcribed LTR elements were identified in the kidney tissues of the NIH-Miniature pig. These elements were divided into two major groups (I and II), and four minor groups (I-1, I-2, I-3, and II-1), by the presence of insertion and deletion (INDEL) sequences. Group I elements showed strong transcriptional activity compared to group II elements. Four different LTR elements (PL1, PL2, PL3, and PL4) as representative of the groups were analyzed by using a transient transfection assay. The regulation of their promoter activity was investigated by treatment with M.SssI (CpG DNA methyltransferase) and garcinol (histone acetyltransferase inhibitor). The transcriptional activity of PERV LTR elements was significantly reduced by treatment with M.SssI. These data indicate that transcribed PERV LTR elements harbor sufficient promoter activity to regulate the transcription of a single virus, and the transcriptional activity of PERV LTRs may be controlled by DNA methylation events.

Structure of Rev-erbalpha bound to N-CoR reveals a unique mechanism of nuclear receptor-co-repressor interaction

Repression of gene transcription by the nuclear receptor Rev-erbalpha plays an integral role in the core molecular circadian clock. We report the crystal structure of a nuclear receptor-co-repressor (N-CoR) interaction domain 1 (ID1) peptide bound to truncated human Rev-erbalpha ligand-binding domain (LBD). The ID1 peptide forms an unprecedented antiparallel beta-sheet with Rev-erbalpha, as well as an alpha-helix similar to that seen in nuclear receptor ID2 crystal structures but out of register by four residues. Comparison with the structure of Rev-erbbeta bound to heme indicates that ID1 peptide and heme induce substantially different conformational changes in the LBD. Although heme is involved in Rev-erb repression, the structure suggests that Rev-erbalpha could also mediate repression via ID1 binding in the absence of heme. The previously uncharacterized secondary structure induced by ID1 peptide binding advances our understanding of nuclear receptor-co-repressor interactions.

hnRNP Q and PTB modulate the circadian oscillation of mouse Rev-erb alpha via IRES-mediated translation

The physiological and behavioral circadian rhythms of most creatures are controlled by a harmony of functional relationships between clock genes. In mammals, several core clock genes show rhythmic profiles of their mRNA and protein expression. Among them, Rev-erb α functions as a transcriptional repressor, affecting expression patterns of other clock genes. For the continuous and robust oscillation of the molecular clock system, the levels of Rev-erb α protein are expected to be tightly regulated with the correct timing. Here, we demonstrate that Rev-erb α has an internal ribosomal entry site (IRES) in its 5′ untranslated region. Furthermore, we demonstrate that heterogeneous nuclear ribonucleoprotein Q and polypyrimidine tract-binding protein (PTB) modulate the IRES-mediated translation of Rev-erb α. We suggest that the rhythmic binding affinity of hnRNP Q to the Rev-erb α IRES and the change in PTB cytosolic levels lead to maintenance of the oscillation profile of the Rev-erb α protein.

Nuclear receptor Rev-erbalpha: a heme receptor that coordinates circadian rhythm and metabolism

Nuclear receptor Rev-erbα (NR1D1), previously considered to be an orphan nuclear receptor, is a receptor for heme, which promotes transcriptional repression via recruitment of the NCoR-HDAC3 corepressor complex. Rev-erbα gene regulation is circadian, and Rev-erbα comprises a critical negative limb of the core circadian clock by directly repressing the expression of the positive clock component, Bmal1. Rev-erbα also regulates the metabolic gene pathway, thus serving as a heme sensor for coordination of circadian and metabolic pathways.

Regulation of metabolism by nuclear hormone receptors

The worldwide epidemic of metabolic disease indicates that a better understanding of the pathways contributing to the pathogenesis of this constellation of diseases need to be determined. Nuclear hormone receptors comprise a superfamily of ligand-activated transcription factors that control development, differentiation, and metabolism. Over the last 15 years a growing number of nuclear receptors have been identified that coordinate genetic networks regulating lipid metabolism and energy utilization. Several of these receptors directly sample the levels of metabolic intermediates and use this information to regulate the synthesis, transport, and breakdown of the metabolite of interest. In contrast, other family members sense metabolic activity via the presence or absence of interacting proteins. The ability of these nuclear receptors to impact metabolism and inflammation will be discussed and the potential of each receptor subfamily to serve as drug targets for metabolic disease will be highlighted.

Negative feedback maintenance of heme homeostasis by its receptor, Rev-erbalpha

Intracellular heme levels must be tightly regulated to maintain proper mitochondrial respiration while minimizing toxicity, but the homeostatic mechanisms are not well understood. Here we report a novel negative feedback mechanism whereby the nuclear heme receptor Rev-erbalpha tightly controls the level of its own ligand. Heme binding to Rev-erbalpha recruits the NCoR/histone deacetylase 3 (HDAC3) corepressor complex to repress the transcription of the coactivator PGC-1alpha, a potent inducer of heme synthesis. Depletion of Rev-erbalpha derepresses PGC-1alpha, resulting in increased heme levels. Conversely, increased Rev-erbalpha reduces intracellular heme, and impairs mitochondrial respiration in a heme-dependent manner. Consistent with this bioenergetic impairment, overexpression of Rev-erbalpha dramatically inhibits cell growth due to a cell cycle arrest. Thus, Rev-erbalpha modulates the synthesis of its own ligand in a negative feedback pathway that maintains heme levels and regulates cellular energy metabolism.

The structural basis of gas-responsive transcription by the human nuclear hormone receptor REV-ERBbeta

Heme is a ligand for the human nuclear receptors (NR) REV-ERBalpha and REV-ERBbeta, which are transcriptional repressors that play important roles in circadian rhythm, lipid and glucose metabolism, and diseases such as diabetes, atherosclerosis, inflammation, and cancer. Here we show that transcription repression mediated by heme-bound REV-ERBs is reversed by the addition of nitric oxide (NO), and that the heme and NO effects are mediated by the C-terminal ligand-binding domain (LBD). A 1.9 A crystal structure of the REV-ERBbeta LBD, in complex with the oxidized Fe(III) form of heme, shows that heme binds in a prototypical NR ligand-binding pocket, where the heme iron is coordinately bound by histidine 568 and cysteine 384. Under reducing conditions, spectroscopic studies of the heme-REV-ERBbeta complex reveal that the Fe(II) form of the LBD transitions between penta-coordinated and hexa-coordinated structural states, neither of which possess the Cys384 bond observed in the oxidized state. In addition, the Fe(II) LBD is also able to bind either NO or CO, revealing a total of at least six structural states of the protein. The binding of known co-repressors is shown to be highly dependent upon these various liganded states. REV-ERBs are thus highly dynamic receptors that are responsive not only to heme, but also to redox and gas. Taken together, these findings suggest new mechanisms for the systemic coordination of molecular clocks and metabolism. They also raise the possibility for gas-based therapies for the many disorders associated with REV-ERB biological functions.

The nuclear receptor Rev-erbalpha is a liver X receptor (LXR) target gene driving a negative feedback loop on select LXR-induced pathways in human macrophages

A role of the nuclear receptor Rev-erbalpha in the regulation of transcription pathways involving other nuclear receptors is emerging. Indeed, Rev-erbalpha is a negative regulator of transcription by binding to overlapping response elements shared with various nuclear receptors, including the peroxisome proliferator-activated receptors and the retinoid-related orphan receptor alpha (RORalpha). Here, we show that Rev-erbalpha is expressed in primary human macrophages and that its expression is induced by synthetic ligands for the liver X receptors (LXRs), which control cholesterol homeostasis, inflammation, and the immune response in macrophages. LXRalpha binds to a specific response element in the human Rev-erbalpha promoter, thus inducing Rev-erbalpha transcriptional expression. Interestingly, Rev-erbalpha does not influence basal or LXR-regulated cholesterol homeostasis. However, Rev-erbalpha overexpression represses the induction of toll-like receptor (TLR)-4 by LXR agonists, whereas Rev-erbalpha silencing by short interfering RNA results in enhanced TLR-4 expression upon LXR activation. Electrophoretic mobility shift, chromatin immunoprecipitation, and transient transfection experiments demonstrate that Rev-erbalpha represses human TLR-4 promoter activity by binding as a monomer to a RevRE site overlapping with the LXR response element site in the TLR-4 promoter. These data identify Rev-erbalpha as a new LXR target gene, inhibiting LXR-induction of TLR-4 in a negative transcriptional feedback loop, but not cholesterol homeostasis gene expression.

Identification of heme as the ligand for the orphan nuclear receptors REV-ERBalpha and REV-ERBbeta

The nuclear receptors REV-ERBalpha (encoded by NR1D1) and REV-ERBbeta (NR1D2) have remained orphans owing to the lack of identified physiological ligands. Here we show that heme is a physiological ligand of both receptors. Heme associates with the ligand-binding domains of the REV-ERB receptors with a 1:1 stoichiometry and enhances the thermal stability of the proteins. Results from experiments of heme depletion in mammalian cells indicate that heme binding to REV-ERB causes the recruitment of the co-repressor NCoR, leading to repression of target genes including BMAL1 (official symbol ARNTL), an essential component of the circadian oscillator. Heme extends the known types of ligands used by the human nuclear receptor family beyond the endocrine hormones and dietary lipids described so far. Our results further indicate that heme regulation of REV-ERBs may link the control of metabolism and the mammalian clock.

The orphan nuclear receptor Rev-erbbeta recruits Tip60 and HDAC1 to regulate apolipoprotein CIII promoter

Nuclear hormone receptors function as ligand activated transcription factors. Ligand binding and modification such as acetylation have been reported to regulate nuclear hormone receptors. The orphan receptors, Rev-erbalpha and Rev-erbbeta, are members of the nuclear receptor superfamily and act as transcriptional repressors. In this study, the role of recruitment of co-factors by Rev-erbbeta and acetylation of Rev-erbbeta in modulating apolipoprotein CIII (apoCIII) transcription were investigated. Rev-erbbeta was found to transcriptionally repress apoCIII after binding to the apoCIII promoter. Tip60, a histone acetyl-transferase (HAT), was a novel binding partner for Rev-erbbeta and recruited to the apoCIII promoter by Rev-erbbeta. Tip60 was able to acetylate Rev-erbbeta and relieve the apoCIII repression mediated by Rev-erbbeta. This de-repression effect depended on acetylation of Rev-erbbeta at its RXKK motif by Tip60. In addition, histone deacetylase 1 (HDAC1) interacted with Rev-erbbeta and was recruited to the apoCIII promoter by Rev-erbbeta to antagonize Tip60's activity. Taken together, we have provided evidence that Rev-erbbeta modulates the apoCIII gene expression by recruiting different transcription co-activator or co-repressor.

A zinc finger HIT domain-containing protein, ZNHIT-1, interacts with orphan nuclear hormone receptor Rev-erbbeta and removes Rev-erbbeta-induced inhibition of apoCIII transcription

The orphan receptors, Rev-erbalpha and Rev-erbbeta, are members of the nuclear receptor superfamily and specifically repress apolipoprotein CIII (apoCIII) gene expression in rats and humans. Moreover, Rev-erbalpha null mutant mice have elevated very low density lipoprotein triacylglycerol and apoCIII levels. However, ligands for Rev-erb are unknown and the regulatory mechanism of Rev-erb is poorly understood. Conceivably, cofactors for Rev-erb may play an important role in the regulation of lipid metabolism. In this study, a zinc finger HIT domain-containing protein, ZNHIT-1, interacted with Rev-erbbeta. ZNHIT-1 was found to be a conserved protein in eukaryotes and was highly abundant in human liver. Furthermore, ZNHIT-1 was identified as a nuclear protein. Serial truncated fragments and substitution mutations established a putative nuclear localization signal at amino acids 38-47 of ZNHIT-1. A putative ligand-binding domain of Rev-erbbeta and the FxxLL motif of ZNHIT-1 were required for their interaction. Finally, ZNHIT-1 was recruited by Rev-erbbeta to the apoCIII promoter and removed the Rev-erbbeta-induced inhibition of apoCIII transcription. These findings demonstrate that ZNHIT-1 functions as a cofactor to regulate the activity of Rev-erbbeta, and may play a role in lipid metabolism.

International Union of Pharmacology. LXVI. Orphan nuclear receptors

Half of the members of the nuclear receptors superfamily are so-called "orphan" receptors because the identity of their ligand, if any, is unknown. Because of their important biological roles, the study of orphan receptors has attracted much attention recently and has resulted in rapid advances that have helped in the discovery of novel signaling pathways. In this review we present the main features of orphan receptors, discuss the structure of their ligand-binding domains and their biological functions. The paradoxical existence of a pharmacology of orphan receptors, a rapidly growing and innovative field, is highlighted.

Elovl3: a model gene to dissect homeostatic links between the circadian clock and nutritional status

The ELOVL3 protein is a very long-chain fatty acid elongase found in liver, skin, and brown adipose tissues. Circadian expression of the Elovl3 gene in the liver is perturbed in mutant CLOCK mice but persists in mice with severe hepatic dysfunction. A reliance on an intact clock, combined with the refractoriness to liver decompensation and the finding of a robust sexually dimorphic pattern of expression, evince a particularly complex mode of transcriptional control. The Elovl3 gene upstream region was repressed by RevErbalpha and activated by sterol-regulatory element binding protein-1 (SREBP1) transcription factors. We propose that the temporal coordination of RevErbalpha and SREBP1 activities integrates clock and nutrition signals to drive a subset of oscillatory transcripts in the liver. Proteolytic activation of SREBP1 is circadian in the liver, and because the cycle of SREBP1 activation was reversed after restricting meals to the inactive phase of the day, this factor could serve as an acute sensor of nutritional state. SREBP1 regulates many known lipogenic and cholesterogenic circadian genes; hence, our results could explain how feeding can override brain-derived entraining signals in the liver. This mechanism would permit a rapid adjustment in the sequence of key aspects of the absorptive and postabsorptive phases in the liver.

The orphan nuclear receptor Rev-erb alpha regulates circadian expression of plasminogen activator inhibitor type 1

Plasminogen activator inhibitor type 1 (PAI-1) is a major physiologic regulator of the fibrinolytic system and has recently gained recognition as a modulator of inflammation and atherosclerosis. PAI-1 exhibits circadian rhythmicity in its expression, peaking in the early morning, which is associated with increased risk for cardiovascular events. However, the mechanisms that determine PAI-1 circadian rhythmicity remain poorly understood. We discovered that the orphan nuclear receptor Rev-erb alpha, a core component of the circadian loop, represses human PAI-1 gene expression through two Rev-erb alpha binding sites in the PAI-1 promoter. Mutations of these sites, as well as RNA interference targeting endogenous Rev-erb alpha and its corepressors, led to increased expression of the PAI-1 gene. Furthermore, glycogen synthase kinase 3beta (GSK3beta) contributes to pai-1 repression by phosphorylating and stabilizing Rev-erb alpha protein, which can be blocked by lithium. Interestingly, serum shock generated circadian oscillations in PAI-1 mRNA in NIH3T3 cells, suggesting that PAI-1 is a direct output gene of the circadian loop. Ectopic expression of a stabilized form of Rev-erb alpha that mimics GSK3beta phosphorylation dramatically dampened PAI-1 circadian oscillations. Thus, our results suggest that Rev-erb alpha is a major determinant of the circadian PAI-1 expression and a potential modulator of the morning susceptibility to myocardial infarction.

The orphan Rev-erb nuclear receptors: a link between metabolism, circadian rhythm and inflammation?

Nuclear hormone receptors (NRs) function as ligand dependent DNA binding proteins that translate physiological/nutritional signals into gene regulation. Dysfunctional NR signaling leads to many disorders in reproduction, inflammation, and metabolism. The opportunity to identify novel regulatory pathways in the context of human health and disease drives the challenge to unravel the biological function of the “orphan nuclear hormone receptors”. For example, the Rev-erb (NR1D) subgroup (Rev-erbα/NR1D1 and Rev-erbβ/NR1D2) of orphan NRs are transcriptional silencers and negative regulators of RORα mediated trans-activation. The NR1D subgroup is highly enriched in peripheral tissues with onerous energy demands including skeletal muscle, brown and white adipose, brain, liver and kidney. This alludes to the involvement of this subgroup in metabolism. In this context, Rev-erbα-/- mice have a dyslipidemic phenotype. Recent studies in vascular smooth and skeletal muscle cells also suggest that the NR1D subgroup modulates inflammation by regulating IκBα/NFκB dependent gene expression. Rev-erbα has been identified as a critical regulator (and target) of circadian rhythm, a factor in blood pressure control and inflammation. Finally, two recent reports have demonstrated: (i) lithium mediated regulation of Rev-erbα stability and (ii) E75 (the Drosophila orthologue of human Rev-erbα) is tightly bound by heme, and functions as a “gas sensor” through interaction with CO/NO and interferes with the repression of DHR3 (the Drosophila orthologue of human RORα). In conclusion, the role of these receptors at the cross-roads of metabolism, inflammation, and circadian cycling underscores the importance of understanding the organ-specific function of the NR1D subgroup in homeostasis.